Sydney's climate history, or the climatic history of the Sydney region, encompasses changes in the climate of Sydney over geological and historical timescales. Palaeoclimatic evidence indicates substantial changes in temperature, precipitation and vegetation in the region, ranging from glacial conditions during the late Palaeozoic to the warmer climates of the Mesozoic and Cenozoic, followed by considerable climatic variability during the Quaternary. Little direct evidence is available for reconstructing the climate of the Sydney region during the Ordovician, Silurian and Devonian, and consequently its climatic conditions during these periods remain poorly constrained.
Written descriptions of Sydney's climate date from the beginning of European settlement in the late 18th century. In 1788, Lieutenant Ralph Clark, a member of the First Fleet, stated that the thunderstorms were the most terrible he has ever experienced. In 1819, Australian explorer William Wentworth described the summer heat as "sometimes excessive" and "oppressive to Europeans", although he noted that sea breezes effectively moderated temperatures and the climate was overall "highly salubrious".Statistical, Historical, and Political Description of NSW by William Charles Wentworth, 1819 In the February 1938 issue of The Home, journalist Basil Burdett wrote, "...Even Melbourne seems like some grey and stately city of Northern Europe compared with Sydney's sub-tropical splendours." Palaeoclimate Late Palaeozoic glaciation
During the Late Carboniferous, south-eastern Australia experienced a cold high-latitude climate associated with the Late Paleozoic Ice Age. The earliest preserved evidence from the Sydney region indicates glacial conditions, with tundra-like vegetation colonising landscapes affected by glaciation. As conditions subsequently became less severe, vegetation expanded and the polar tree line advanced into the region. Cold conditions persisted into the Early Permian, when the Sydney region lay at a high southern palaeolatitude. The Sydney Basin preserves evidence of glacial influence including glacial outwash, dropstones, ice-keel scours and glendonites. The ice-keel scours indicate that floating ice occurred at least seasonally, while glendonites have been interpreted as evidence of near-freezing bottom waters. In the late Early Permian Wandrawandian Siltstone, glendonites represent pseudomorphs after ikaite, a mineral associated with cold marine sediments and temperatures of approximately 0–7 °C.
Direct estimates of air temperature are uncertain, and precise mean air temperatures for the Sydney region during these glacial phases remain unknown. Quantitative estimates from the oxygen-isotope composition of Permian fossil brachiopod shells provide further evidence of a cool high-latitude climate. Using an assumed seawater isotopic value of −1‰, estimated temperatures for the Sydney Basin were predominantly about 12–16 °C during the Early and Middle Permian, although considerably warmer apparent temperatures were obtained for some intervals. Changes in local seawater composition, including the influence of glacial meltwater, may have affected these isotope-derived temperatures. Glaciation in eastern Australia during the Permian was intermittent rather than continuous, with glacial intervals separated by relatively warmer phases, and glacial influence persisted considerably later than in many other parts of Gondwana.
As the Late Paleozoic Ice Age waned, the climate became progressively milder. Gangamopteris vegetation was succeeded by extensive Glossopteris forests characteristic of the cool-temperate regions of Gondwana. By the Late Permian, humid conditions supported extensive deciduous Glossopteris swamp forests and the accumulation of coal. Palaeosols associated with the uppermost Permian coal measures have been interpreted as representing cold-temperate wetlands, with some features comparable to modern high-latitude peatlands. Immediately before the Permian–Triassic extinction event, approximately 252 million years ago, climate simulations for the Sydney Basin indicate a strongly seasonal continental climate, with warm, wet summers and cold, comparatively dry winters. The modelled summer–winter temperature difference was approximately , with standard simulations indicating mean temperatures below freezing during the coldest part of winter. The end of the Permian was accompanied by a major climatic disturbance and the disappearance of the Glossopteris flora from the Sydney Basin. Palaeosol and weathering evidence does not indicate an abrupt transition to arid conditions; climate modelling instead suggests moderately warmer summers and increased temperature seasonality across the extinction interval.
Triassic and Cretaceous greenhouse climates The Early Triassic experienced a much warmer greenhouse climate, with modelling indicating warmer and wetter conditions during all seasons and particularly pronounced warming of the previously cold winters. Palaeosols from the Sydney Basin similarly record enhanced chemical weathering. Some indicate mean annual temperatures of at least approximately 11 °C and annual precipitation exceeding 1,100 mm, comparable with modern humid temperate climates at substantially lower latitudes. Conditions were nevertheless variable: five successive episodes of elevated atmospheric carbon dioxide and intense chemical weathering have been identified in the Sydney Basin, representing transient greenhouse crises separated by generally cooler and drier intervals.
Estimates from palaeosol chemistry suggest annual precipitation of approximately 1,000–1,300 mm around the Permian–Triassic extinction event, declining to approximately 800–1,200 mm during deposition of the lower Narrabeen Group. Sedimentary and palaeobotanical evidence also indicates increasing seasonality and periods of moisture deficiency, although conditions did not become fully arid. The climatic transition was accompanied by major vegetation changes: the deciduous Glossopteris forests disappeared and small-leaved evergreen plants became more prominent. Palaeosols indicate that forests, heaths and coastal wetlands subsequently developed as vegetation recovered and diversified through the Triassic. Later Triassic plant assemblages indicate that the Sydney region remained generally humid and temperate, although climatic conditions varied through the period.
Direct evidence for Sydney's Cretaceous climate is limited. Regional Early Cretaceous palaeobotanical evidence from south-eastern Australia, which remained connected to Antarctica and lay at considerably higher southern latitudes than at present, indicates a cool and humid climate supporting extensive high-latitude forests despite prolonged winter darkness. Moisture-dependent vegetation on river floodplains indicates generally humid conditions and an absence of widespread aridity. By the Late Cretaceous, conditions in south-eastern Australia were substantially warmer. Reconstructions at palaeolatitudes of approximately 60–65°S indicate mean annual temperatures of about , despite the strongly seasonal daylight regime. Warm, humid conditions supported extensive forests and fern-rich wetlands, indicating abundant moisture in at least parts of the landscape.
Cenozoic cooling and drying During the Palaeogene, south-eastern Australia remained substantially wetter and more heavily forested than at present. Late Palaeocene fossil pollen and wood from southern New South Wales indicate a humid climate supporting cool-temperate rainforest. Climatic estimates derived from these assemblages suggest mean annual temperatures broadly between , with mild winters and little evidence of significant frost.
Regional evidence indicates that the Eocene was considerably warmer. Palaeobotanical reconstructions from early and middle Eocene southern Australia indicate a generally subtropical climate, with mean annual temperatures of approximately , mean temperatures during the warmest quarter of and during the coldest quarter of . Coldest-quarter temperatures generally remained above , suggesting that frost was exceedingly rare despite southern Australia's substantially higher palaeolatitude. Rainfall varied geographically, with coastal sites generally receiving more than annually and some wetter sites exceeding , whereas inland sites averaged approximately . Rainfall was probably seasonal, with the driest month receiving approximately two to seven times less precipitation than the monthly average. Despite this seasonality, high temperatures and an intensified hydrological cycle supported generally humid conditions across much of southern Australia.
A major cooling occurred near the Eocene–Oligocene transition, associated with declining atmospheric carbon dioxide concentrations and the development of large Antarctic ice sheets. Terrestrial records from south-eastern Australia indicate substantial cooling during the Late Eocene and a further temperature decline across the Eocene–Oligocene boundary. Pollen records spanning approximately 35.5 to 23 million years ago indicate that the climate nevertheless remained predominantly mesothermic and humid. Reconstructed mean annual temperatures were generally about , averaging approximately , but fell to around during the Eocene–Oligocene cooling. Annual precipitation remained high at approximately , averaging about . The Oligocene climate was not uniformly cool: several warming cycles followed the transition, including a prolonged warmer interval between approximately 32 and 26 million years ago during which reconstructed mean annual temperatures reached . Increasing climatic seasonality developed towards the end of the Oligocene.
During the Miocene, south-eastern Australia experienced a long-term decline in precipitation and contraction of rainforest. Around 17 million years ago, Sydney lay at a palaeolatitude of approximately 45–50°S. Extensive lateritic and Ultisol-type palaeosols around the region indicate substantial chemical weathering under the Miocene climate. Increasing aridity and rainfall seasonality through the later Miocene contributed to the expansion of more open and drought-tolerant vegetation at the expense of rainforest. Rainfall briefly increased during the early Pliocene, allowing some expansion of rainforest vegetation, before renewed drying during the middle and late Pliocene. Relatively moist conditions nevertheless persisted in parts of south-eastern Australia into the Early Pleistocene. Fossil beetle assemblages from approximately 1.84 to 1.56 million years ago indicate temperatures around warmer than at present and annual precipitation ranging from similar to substantially greater than modern levels. Winter rainfall was approximately comparable with that of today, whereas summer rainfall was about 2–2.4 times greater. The modern strongly seasonal rainfall regime of southern Australia was therefore not yet fully established by approximately 1.5 million years ago.
Late Quaternary climate variability Palaeoclimatic evidence indicates that Sydney's climate underwent substantial variations during the late Pleistocene. Records across the Sydney Basin indicate relatively dry conditions around the Last Glacial Maximum and continuing into the late Pleistocene, including during the Antarctic Cold Reversal. Conditions became generally wetter from approximately 14,500 years ago, although this trend was interrupted by renewed dry phases.